Conventionally, for example, an image transfer system for wirelessly transferring captured image data between an image transmitter and an image receiver by using a high-speed wireless communication technique typified by that of a wireless communication standard such as Institute of Electrical and Electronics Engineers (IEEE) 802.11, i.e., so-called WiFi (registered trademark), has been practically used. In the image transfer system, the image transmitter transmits captured image data obtained through photographing performed by an imaging unit provided in the image transmitter to the image receiver and the image receiver causes a display unit provided in the image receiver to display an image according to the transmitted captured image data.
In such an image transfer system, each of the image transmitter and the image receiver generates a synchronization signal such as a vertical synchronization signal based on, for example, a reference clock signal generated by a crystal oscillation IC or the like, and operates in accordance with a timing of the generated synchronization signal. In other words, in the image transmitter, the image is captured by the imaging unit in accordance with the timing of the synchronization signal generated in the image transmitter. Also, in the image receiver, an image according to the transmitted captured image data is displayed on the display unit in accordance with the timing of the synchronization signal generated in the image receiver.
Meanwhile, in wireless communication, for example, a delay in wireless transfer changes due to an influence such as radio signal interference and a change in an environment between the image transmitter and the image receiver which perform wireless communication. In other words, in wireless communication, a transfer time when captured image data is wirelessly transferred changes. Thus, in the image transfer system, the captured image data is not always transferred at the same timing (cycle) even after a wireless connection between the image transmitter and the image receiver is established and a time until the captured image data transmitted by the image transmitter reaches the image receiver changes. A change in the arrival time of the captured image data becomes a cause of disturbing a timing (a cycle) of displaying an image of each frame according to captured image data received by the image receiver, for example, when the image transmitter transmits captured image data of consecutive frames such as moving images. The disturbance of the timing (cycle) of displaying the images of the frames becomes a cause of making moving images displayed by the image receiver unnatural.
Therefore, for example, as disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-136125, the technology of an image transfer device that performs feedback control so that a change in an arrival time of captured image data is minimized has been proposed. In the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-136325, an image transmitter adds a vertical synchronization marker to the captured image data and transmits the captured image data and an image receiver monitors whether or not a position of the vertical synchronization marker added to the received captured image data is in a predetermined range. In the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-136325, when the position of the monitored vertical synchronization marker is not in the predetermined range, the image receiver transmits an amount of synchronization adjustment for adjusting the timing to the image transmitter so that the position of the vertical synchronization marker is in the predetermined range. Thereby, in the image transfer device disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-136325, it is conceivable that the position of the vertical synchronization marker monitored by the image receiver can be feedback-controlled, and the image transmitter can transmit the captured image data at a similar timing. In other words, in the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-136325, it is considered that the image transfer device in winch the image transmitter and the image receiver are synchronized can be implemented.
However, in the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-136325, the change in the arrival time of the captured image data in the wireless communication is determined on the basis of the vertical synchronization marker added to be captured image data and the image transmitter is synchronized with the image receiver. Thus, in the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-136325, for example, even when the arrival time of only specific captured image data suddenly increases, a timing at which the image transmitter transmits captured image data is adjusted. Also, in the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-136325, when a case in which data of only the vertical synchronization marker is wirelessly transferred at a predetermined timing (cycle) is considered, the arrival time of data of only the vertical synchronization marker is not necessarily short, and, in contrast, the arrival time of only the data of only the vertical synchronization marker may be suddenly prolonged. In this case, in the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-136325, the image transmitter may adjust the timing at which the captured image data is transmitted even if the arrival time of the captured image data is not prolonged.
In other words, in theory, in the image transfer system using wireless communication technology, even if it is not necessary to adjust a timing at which the captured image data is transmitted in response to a sudden change in the arrival time, the timing adjustment is performed in accordance with the sudden change in the arrival time in the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-136325. In other words, in the technology disclosed in Japanese Unexamined Patent Application, First Publication No. 2010-136325, the image transfer system may sensitively react to the change in the arrival time and frequently perform unnecessary timing adjustment in theory. Frequent timing adjustment according to sudden change in the arrival time becomes a cause of increasing a processing load in the image transfer system. Also, in the image transfer system in which the image transmitter transmits captured image data of consecutive frames (for example, moving images), frequent timing adjustment according to a sudden change in the arrival time becomes a cause of disturbance of an imaging timing (cycle) in the imaging unit provided in the image transmitter, i.e., disturbance of moving images.